Staircase



FIG. 1 is a top front perspective view of a staircase showing our new design;

FIG. 2 is a rear view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof. 

CLAIM The ornamental design for a staircase, as shown and described. 